GB-B-2,068,544, discloses a method and apparatus in which the properties of a dilute sample of red blood cells are changed by osmosis to generate a set of related data which is characteristic of the health or physiological condition of the human or animal source of the blood cell sample. By subjecting cells to a number of solutions having different concentrations, the size and shape of the blood cells changes. When these changes are measured electronically, a detected electrical resistance increases to a peak and then falls as the critical concentration passes.
In particular, in GB-B-2,068,544 it is known to be desirable to obtain a series of hypotonic concentrations and thereby measure the characterisetics of blood cells at a series of different osmalolities. In the past, a conventional stepper motor has been used in combination with a lead-screw apparatus to drive a plunger of a first syringe containing a dilutent which is subsequently mixed with a different dilutent contained in a second syringe, the plunger of which is driven by a second stepper motor. A third syringe containing a prepared blood cell sample is discharged at a constant rate into a mixing chamber into which the first and second syringes simultaneously discharge their contents. During the test, the flow rate of fluid from the first syringe is typically decreased over a period of time whilst the flow rate of fluid from the second syringe is correspondingly increased over the same period to change the relative concentrations of the two dilutents whilst size, shape and count measurements are made of the blood cells in the resulting mixture. However, conventional stepper motors can only be driven at a limited number of predetermined stepping rates so that the velocity of fluid discharged from each syringe can only be changed in a stepwise manner, which therefore only very roughly approximates to a desired velocity profile. Each stepwise change in velocity of the conventional stepper motor leads to disturbances in the flow of fluid which affects the accuracy of the size, shape and count measurements made downstream. Accordingly, the accuracy of this conventional arrangement for determining cell osmalolity is only to within 2-3 mosm Kg.sup.-1, which is not sufficiently sensitive to detect certain health or physiological conditions.